This invention relates to an improved apparatus for measuring high boiling components found in liquid streams and particularly existent gum levels found in aviation fuels.
A variety of methods has been used in the analysis of petroleum hydrocarbons including gas chromatography, mass spectrometry, infrared spectrometry, ultraviolet spectrometry, X-ray analysis including X-ray fluorescence, X-ray diffraction and X-ray absorption, and electron microscopy. A number of different instruments have been developed using one or more of these or other methods.
Historically, existent gum (evaporation residue) levels in aviation fuel have been measured because there tends to be a relationship between the levels of existent gum and fuel purity. An increase in these gum levels from the refinery certification level usually means product contamination has occurred and the fuel may contain an unacceptable amount of "heavy ends".
The current technique for measuring existent gum in fuels is the steam jet method described in ASTM D381-70. In this method a measured quantity of fuel is evaporated under controlled conditions of temperature and flow of air or steam. The resulting residue in aviation gasoline and aircraft turbine fuel is weighed and reported as milligrams per 100 ml while for motor gasoline, the residue is weighed before and after extracting with n-heptane and the results reported in milligrams per 100 ml. While this method does provide an adequate and reliable determination of existent gum levels in fuels, it is somewhat cumbersome and inconvenient to run since it involves relatively large pieces of equipment requiring substantial capital investment as well as needing technically skilled personnel to carry out. Because of this, the test generally cannot be performed at individual field locations or terminals but at a central laboratory which involves the added cost of space and storage as well as a loss of time.
Accordingly, there is a need for a simplified, quicker and less complicated technique for measuring existent gum levels in fuels which will require less burdensome apparatus with respect to size and investment. The availability of such a technique and apparatus will permit testing at various locations in the field rather than at a central laboratory and therefore ease the problem of determining off spec quality and allow for quicker correction of such problem at a considerable saving of time and cost.
The use of piezoelectric crystals for the selective analyses of fluid mixtures has been known in the art for some time. The basic principle involved in using a piezoelectric crystal as a detection device involves measuring the mass change of a vibrating crystal. There are two modes in which the crystal is employed. Most involve the use of a predeposited substrate on the crystal to absorb the material to be analyzed. As the substrate or coating of the crystal interacts with another material and thus changes weight, the change in weight or mass can be detected and used for determining qualitatively and quantitatively various components present. One of the early patents which disclosed coated piezoelectric analyzers is U.S. Pat. No. 3,164,004 by William H. King, issued Jan. 5, 1965. Another application of this technique is disclosed in U.S. Pat. No. 3,427,864 by William H. King, issued Feb. 18, 1969, wherein the presence of moisture in the fluid mixture is detected using a piezoelectric crystal coated with a deliquescent salt such as lithium chloride. A variation of this technique is disclosed in U.S. Pat. No. 3,260,104, issued July 12, 1966, and U.S. Pat. No. 3,266,291, issued Aug. 16, 1966, both by William H. King, which show an analyzer having two detection devices both having a piezoelectric crystal, one of which has a substrate selective to a particular material, and the other acting as a reference. The net output of the two detection devices is a measure of the interaction of at least one component to be detected.
Another method of using piezoelectric crystals as detection devices involves applications where the crystal is not coated with a substrate. This generally involves depositing a material mixed in solvent or other carrier on the crystal, evaporation of the carrier leaving behind the material of interest followed by measurement of crystal frequency. Applications of this type have been disclosed in U.S. Pat. No. 3,856,466, issued on Dec. 24, 1974 to Harry M. Crawford, and U.S. Pat No. 3,863,495, issued on Feb. 4, 1975 to Wolfgan Schultz et al.
In the above-described techniques of using piezoelectric crystals, the sample location is dependent in the first instance on the substrate coating amount and dimensions and the makeup of the surrounding materials while in the second instance it is dependent on the particular technique of sample application and control thereof.
Despite the longstanding use of piezoelectric crystals in detection devices as illustrated in the above-noted patents, there still is the need for an apparatus and technique wherein sample location is readily controlled resulting in quick, accurate and consistent measurements of high-boiling components in liquids, particularly existent gum levels in fuels. More particularly, there is a need for a simplified, portable type apparatus useful in measuring gum levels in fuels in a quick manner at convenient locations.